The present invention relates to a surface acoustic wave (SAW) device; and more particularly, to such a device that converts a large amplitude video impulse (or a short burst of sine waves) to a phase coded pulse, or decodes such phase coded pulse to generate a large amplitude short pulse.
Surface acoustic wave (SAW) devices, which convert electrical energy into acoustic energy, and reconvert acoustic energy into electrical energy are well known. Such surface acoustic wave devices typically comprise metallized interdigital transducer elements formed on the surface of a piezoelectric substrate, which substrate may be quartz or lithium niobate, for example. These devices may be easily fabricated by optically polishing a single surface of the piezoelectric substrate, depositing the metal film on the polished surface, and then forming the transducer elements using standard photolithography techniques.
In connection with radar systems, or communication systems, it is sometimes desirable to transmit pulses which are distinctly coded in order to discriminate against received or echo pulses that are not so coded.
Heretofore, phase coded pulse generators have been constructed from surface acoustic wave devices. Such devices typically utilized what may be termed a bi-phase code wherein the phase modulation changes instantaneously between two states; that is, between zero and 180.degree.for example. In other words, such SAW devices utilized interdigital fingers on the piezoelectric substrate to produce phase reversed output signals resulting in the bi-phase code. For example, U.S. Pat. No. 3,961,290 discloses a programmable phase coded surface wave acoustic device wherein various taps on the interdigital elements or fingers are severed from the common conducting bar to selectively reverse the phase of the surface acoustic wave with each tap constituting a binary bit. Although bi-phase SAW devices accomplished the purposes intended, such bi-phase coding is readily discernible and exhibits a broad output spectrum causing undesirable interference.
In order to overcome the easy identification and the wideband spectrum characteristics of the bi-phase coded generators, it was proposed to utilized a code generator that provided a quadraphase code; that is, that the phase of the pulses shifted 90.degree. rather than 180.degree.; furthermore, the phase was to change gradually during the bit period. Such quadraphase coded generators were frequency shift coded, and were constituted by electronic devices with logic circuitry and modulators. The frequency was shifted from a center, or carrier frequency, utilizing complicated modulating gates. The modulated gates were required to insure a constant output level over the center portion of each pulse. Such electronic quadraphase code generators were not only relatively expensive but they also consumed a substantial amount of power, required a separate shaping filter to control the rise and fall time of the pulses to be coded, and occupied a large volume.
Heretofore, it was also proposed to generate the quadraphase code by coherently impulsing a surface acoustic wave delay line. Such a device consisted of two interleaved 13 bit Barker bi-phase codes which are summed in such a way that one of the codes is in quadrature with the other. By shifting one of the two decoder patterns with respect to the other by one half of bit, and then shifting it again by one quater of a cycle, the essential elements of the quadraphase code are realized once the two patterns are summed, which apparatus is also relatively complicated.
Thus, it is desirable not only to eliminate the complicated electronic encoder to generate the frequency shift codes, but also to provide a simplified SAW device for providing a continuous phase coding with controlled rise and fall times. Further, it is desirable to provide such a SAW device that is capable of being used as an encoder and decoder and may be readily fabricated to generate a plurality of different quadraphase codes with selected rise and fall times of the pulse to be coded.